Two reasons. First, that random generator is only ever going to generate one PDU, and it's not going all the way through the deframer for some reason. I think the deframer is asking for more samples than the framer ever ends up generating. Try a Message Strobe - Random Delay block, with the message set to:
pmt.cons(pmt.PMT_NIL, pmt.init_u8vector(4, [0xAA]*4))
Second, the deframer length fields include the HDLC CRC. They probably shouldn't, in hindsight, but increase the max length to 6 and it'll work.
--n